When semiconductor devices are mounted on a dielectric or magnetic ceramic substrate, soldering is conducted by coating the substrate with a Sn-based solder, mounting the semiconductor devices on the solder, and heating the solder. As semiconductor devices are mounted on a ceramic substrate part at higher density, wire bonding has become used for electric connection between the ceramic substrate part and the semiconductor devices. In high-density wire bonding, gold wires as thick as 100 μm or less are connected to the wire-bonding electrodes of a heated ceramic substrate part with ultrasonic vibration.
The wire-bonding electrode generally comprises a primer layer of silver, copper, etc., a nickel plating layer, a substituted gold plating layer, and a reduced gold plating layer formed in this order on a ceramic substrate. In such terminal structure, the nickel plating layer acts as a barrier layer for protecting the primer layer from a solder. The gold plating layer provides improved connectability with gold wires. Reduction gold plating makes it possible to form a gold layer having any thickness, but because its plating solution is vulnerable to Ni, the substituted gold plating layer should be formed below the reduced gold plating layer.
However, part of the Ni plating layer corroded by substitution gold plating remains as pinholes after reduction gold plating. Accordingly, when the substrate is heated, nickel hydroxide generated by water intruding through the pinholes is exposed to the gold surface, extremely lowering wire bondability. To close the pinholes, an expensive reduced gold plating layer should be formed to a thickness of 0.2-0.7 μm, suffering high cost. In addition, even a thick reduced gold plating layer may not be able to bury the pinholes, resulting in bonding strength deteriorated by heat treatment.
Proposed to solve such problems is a method of forming only a substituted gold plating layer as thick as 0.1 μm or less, and removing nickel hydroxide from a gold layer surface by a plasma treatment before wire bonding. However, the substituted gold plating layer has many pinholes, and nickel much diffused to the gold surface during heat treatment cannot easily be removed by a plasma treatment, resulting in poor bonding reliability. Therefore, this method cannot be used for electronic parts needing high reliability.
JP 2004-55624 A discloses the formation of a Pd layer between a Ni layer and a Au layer to prevent the diffusion of Ni. However, Pd is thermally diffused to the Au layer while soldering devices such as inductors, etc. to mounting electrodes before the wire bonding of semiconductor devices, so that the wire bondability of the Au layer is deteriorated. This is because the existence of other metals than gold deteriorates the strength of bonding between gold wires and a gold plating layer, which is achieved by mutual diffusion with ultrasonic vibration. As lead-free, high-melting-point solders are widely used recently, the diffusion of Pd to a Au layer during soldering has become a serious problem.